Composite Fouling Characteristics of Calcium Oxalate Monohydrate and Amorphous Silica by a Novel Approach Simulating Successive Effects of a Sugar Mill Evaporator

Abstract

The composite fouling of calcium oxalate monohydrate (COM) and amorphous silica (SiO 2 ) were investigated in a dynamic system under subcooled flow-boiling and a novel but simple continuous evaporation setup to simulate the operation cycle in latter effects of sugar mill evaporators. Simulated solutions with various COM to SiO 2 supersaturation (SS) ratios as well as those of pure COM and SiO 2 were tested for comparative studies. The results demonstrated that COM fouling resistances began to rise shortly after the experiment started with the highest extent of fouling exhibited at an initial COM concentration of 50 ppm (initial COM SS ∼ 2.6). The fouling resistance of SiO 2 (500 ppm) began to rise at a theoretical silica supersaturation level of 5.3. SiO 2 fouling involved the deposition of two silica species, that is, dissolved monomeric silica and colloidal silica particles, with colloidal silica particles being more prevalent at SiO 2 supersaturation. In the binary systems, the synergistic effect of COM on composite fouling occurred at an intermediate concentration of COM (50 ppm) whereas antagonism was obtained at either low or high COM concentration (20 or 100 ppm). The observed variations in the extent of composite fouling may be partly attributed both to the changes in the magnitude of interfacial energy barrier between the surface of the particle and the wall and to the physical properties of the fouling species such as particle size. Instrumental analysis (SEM-EDS and XRD) was used to investigate structure and composition of scale. The presence of COM and its cementing effect in the composite scale from [COM] ) 50 ppm were confirmed.

Modeling of calcium oxalate and amorphous silica composite fouling

“…Details of the experimental setup and test conditions used in this work have been provided elsewhere 4. In brief, the closed‐loop circulation system includes a circulation pump; a storage tank; an annular test section (Pegasus, Canada); a cooling heat exchanger; a level control system (Burkert Contromatic Australia P/L); series of pressure, temperature, and flow measuring and control devices; and a PC with a data acquisition and control system (DA&C, Genie, American Advantech Co.).…”

“…The composite fouling of CaOx [as calcium oxalate monohydrate (COM), the most stable hydrate form of CaOx under high temperatures] and SiO 2 in water and sugar solutions has been the subject of a systematic investigation by the authors with the latest work relating the effects of various operating conditions to the mechanisms of composite fouling 4–6. The results indicated that the primary mechanism of composite fouling was particulate deposition of COMSiO 2 colloidal species.…”

Roles of reversible and irreversible aggregation in sugar processing

“…Boiling rate could be improved by adding HCl and H 2 O 2 to the vacuum pan (Eggleston et al, 2011), suggesting the importance of polyaromatic impurities. In addition to organic (macromolecular) impurities, calcium oxalate monohydrate and silica (SiO 2 ) deposit on the surface of evaporator units to cause a similar heat transfer problem (Yu et al, 2002). The inorganic deposit is compact and hard, and is difficult to remove by chemical cleaning (Doherty, 2006).…”

Scale in Sugar Juice Evaporators

“…This equated to over 48% of world exports of raw sugar [1]. Brazil produces approximately 25% of the world's raw sugar, with India, the European Union, and China the next largest producers.More than 80% of industrial heat exchangers (present in oil, gas, chemical, petrochemical, mineral processing, power generation, food, water purification, and pulp and paper industries) suffer from fouling problems [2], which cause inefficiencies, plant shutdowns, and degradation of product and plant equipment [3]. Worldwide costs associated with heat exchanger fouling alone have been estimated at US$26 billion/year, or 0.2% of the gross national product of the industrialized countries [2].…”